In recent years, attention has been focused on methods of high heat flux removal at low surface temperatures. This is due to the advancing requirements of the electronics industry that prevent high temperature heat transfer due to the operating conditions of electronics. Though the heat transfer process is very complex and still not completely understood, many evaporative spray cooling experiments have been performed which indicated the high heat removal capability of this cooling technique. The spray technique generally works in the following way; a spray nozzle is use to atomize a pressurized liquid, and the resulting droplets are impinged onto the heated surface. A thin film of liquid is formed on the heat transfer surface in which nucleate boiling takes place. The droplet impingement simultaneously causes intense convection and free surface evaporation. When a liquid with high latent heat of vaporization (such as water) is used, over 1 kW/cm2 of heat removal capability has been demonstrated.
The temperature of the cooled surface is determined by the boiling point of the liquid. Since the resulting heat transfer coefficient is very large (50,000 to 500,000 W/m2 C) the surface temperature will be only a few ° C. above the boiling point of the liquid.
This type of cooling technique is most appropriately implemented when used to cool high heat flux devices such as power electronics, microwave and radio frequency generators, and diode laser arrays.
As stated above, diode laser arrays and microwave generators are devices that can be cooled with this type of impinging spray technology. Current market forces are driving these devices to increased power and size requirements. As a result, high heat flux devices are now being designed with surface areas much larger than 2 cm2. New high heat flux devices may be 10 cm2 to 1000 cm2, or larger. Spray cooling techniques offer the ability to provide superior cooling if atomizer nozzle and nozzle arrays can be developed that provide optimal cooling characteristics. Typical atomizer nozzles for painting, fuel atomization, and humidification do not meet the cooling need of these next generation high power electronic devices.